Digital data communication systems are currently enjoying widespread use due to their benefits over analog communication systems. Typically, most data communication systems utilize filters in generating the data to be transmitted. Filters are especially used in communication systems that employ modems to transmit digital data over analog transport facilities. Most modems include wave shaping circuitry for generating appropriate signals for transmission over the analog communication channel. Note that the analog communication channel may comprise copper, waveguide, optical fiber, RF, microwave, IR, etc.
The function of the modem is to accept the digital data to be transmitted and generate analog signals suitable therefrom for transmission over the analog communication channel. To perform this function, modems employ wave shaping circuitry to first shape the input digital waveform and subsequently modulate this waveform with a carrier frequency signal. The wave shaping is performed to provide the transmitted signal with spectral characteristics suitable to the particular communication channel the data is to be transmitted on.
The shaping circuitry used in prior art transmission systems typically utilized large amounts of memory storage to store the waveform to be used in shaping the input digital data U.S. Pat. No. 5,548,541, issued to Bierman et al. teaches a finite impulse response (FIR) filter for shaping a one bit serial digital data pulse train. The filter employs a delay element for sequentially receiving binary data bits in the data pulse train at fixed data cycle intervals and outputting simultaneously in parallel a plurality of data bits representing the most recent history of the past N data cycle intervals. Also included is a sample element for sampling the pulse train and a memory device for storing output values.
With this type of system, as the number of samples for each symbol increases, the memory required for storage of the waveform also increases by a large amount. Thus, there is a need for a communication system which reduces the storage requirements for a given number of waveform samples per symbol.